Time modulator for electric pulses



Feb. 27, 11951 G. H. HQUGH ET AL TIME MODULATOR FOR ELECTRIC PULSES Filed Nov, 9, 1948 Dfi/dg A TTOFF/Vj Patented Feb. 27, 1951 TIME MODULATOR FOR ELECTRIC PULSES George Hubert Hough and Thomas Meirion Jackson, London, England, assignors to International Standard Electric Corporation,

New

York, N. Y., a corporation of Delaware Application November 9, 1948, Serial No. 59,082 In Great Britain November 11, 1947 7 Claims.

The present invention relates to pulse time modulating arrangements for electric pulse communcation systems.

The invention employs a cold cathode gas discharge tube, and takes advantage of an ionization delay effect which is explained in the specication of copending U. S. application 763,655, filed July 25, 1947.

The invention accordingly provides an electric pulse time modulator comprising a cold cathode gas discharge tube` having two electrodes with a gap therebetween, means for applying to said electrodes a difference of potential less than the potential necessary for maintaining a discharge across the gap, means for applying a train of pulses to one of the said electrodes, each pulse being of such amplitude as to raise the said difference of potential to a value greater than the potential necessary for producing a discharge across the gap, means for applying a modulating signal wave to one of the electrodes to vary the said difference of potential and means for deriving time modulated pulses from one of the electrodes.

An embodiment of the invention will be described with reference to the accompanying drawing which shows in Fig. 1 a schematic circuit diagram of the embodiment, and in Fig. 2, a curve used in the explanation of the operation of the embodiment.

The embodiment of the invention shown in Fig. 1, comprises a cold cathode gas-filled tube I which contains an anode 2 in the form of a Wire, or rod, or a flat metal plate, a plate or Wire cathode 3 arranged parallel to the anode and an optional auxiliary cathode 4 in the form of a wire or rod arranged opposite the anode 2 at some distance from the cathode 3. It will be understood that the tube is diagrammatically shown, and may be constructed in any suitable manner by conventional technique.

The anode 2 is connected through a resistance 5 to the positive terminal 6 for a high tension source (not shown), the corresponding negative terminal 1 being connected to ground. The cathode 3 is .connected through the primary winding of a pulse transformer 8 and a resistance 9 to a terminal I0 for a first negative bias source BI (not shown). The auxiliary cathode 4 is connected through a resistance II to a terlance I9.

2 minal I2 for a second negative bias source B2 (not shown). The positive terminals of these bias sources will be connected to terminal 1.

A train of regularly repeated rectangular pulses from a source I2a will be applied between an input terminal I3 and a ground terminal I4. Terminal I3 is connected to the anode 2 through a blocking condenser I5. The pulses should be applied in positive sense to the anode.

The modulating signal wave from a, source I5a is applied between terminals I6 and I4. Terminal IB is connected through a blocking condenser I1 to the junction point of the primary` winding of the transformer 8 and resistance 9.

The secondary winding of the transformer 8 is connected to a differentiating circuit comprsing a series condenser4 I8 and a shunt resist- The terminals of the resistance I9 are connected to the input of an amplitude limiter 20, the output of which is connected to a pair of output `terminals 2I and 22. Time phase modulated pulses may be obtained from these terminals.

The potential diierence between the electrodes 2 and 3 should always be lower than the potential necessary to maintain a discharge between these electrodes in the absence of any other applied potentials. The auxiliary cathode I is provided to maintain a permanent discharge to the anode 2 for the purpose of controlling the level of ionization of the tube, as explained in the specification of co-pending U. S. application No. 19,084 led April 5, 1948, now Patent No. 2,520,171. The difference of potential between the electrodes 2 and 3 will be equal to the sum of the potentials of the high tension source, of the bias source BI, and of the applied signal potential, less the potential drop caused by the auxiliary cathode current in the resistance 5. 'Ihe potential of the bias source BI will be relatively small and preferably adjustable but it will be clear that the potential of the bias source B2 will bey rather greater in order to produce and maintain the discharge to the auxiliary cathode 4.

The positive rectangular pulses applied to terminal I3 should be of such amplitude and duration that each of them is able to raise the difference of potential between the electrodes 2 and 3 above the striking voltage and to proananas total difference of potential effective between the electrodes: the higherl this difference of potential, the shorter the delay. This result is illustrated by the curve of Fig. 2 which shows the relation between the delay in striking (ordinates) and the electrode difference of potential (abscissae). It will be noted that the potential of the cathode 3 depends on the instantaneous signal voltage applied at terminal I6. It will be clear therefore that the signal potential will vary the electrode difference of potential between two limits, such as V1 and V2, shown in Fig. 2, and so the delay in the striking time of the discharge will vary between two corresponding limits t1 and tz. Thus the leading edges of the pulses of current passed through the transformer 8 will be time modulated in accordance with the signal wave, but the trailing edges will be substantially unmodulated, since they will coincide with the trailing edges of the pulses applied at terminal I3. It will be clear therefore, that duration modulated pulses will be obtained from the secondary winding of the transformer 8. These duration modulated pulses may be differentiated by the condenser I8 and resistance I9 to produce pairs of short pulses of opposite sign coinciding respectively With the leading and trailing edges of the duration modulated pulses. The trailing edge pulses may be removed by the amplitude limiter 20, leaving the time-phase modulated leading edge pulses to be obtained at terminals 2I and 22. The limiter 20 may be of any suitable type, and could consist for example, of a thermionic valve biassed just below cut-off, the leading edge pulses being applied in positive sense to the control grid. The negative trailing edge pulses would then be eliminated and time modulated negative output pulses would be obtained from the anode. These output pulses could be inverted if necessary in any convenient way. If duration modulated output pulses are required, the elements I 8, I9 and 20 could be omitted, the output terminals 2I and 22 being connected directly to the secondary winding of the transformer 8.

It will be evident that the duration of the pulses applied to terminal I3 should be greater than t1 (Fig. 2), which is the maximum delay which can be produced by the modulating signal, otherwise the discharge will sometimes fail to strike.

The bias source BI may be used to adjust the potential of the cathode 3 so that the electrode difference of potential is just less than the maintaining potential, for the maximum negative signal potential, in order to ensure that the discharge will always be extinguished on the disappearance of each applied pulse.

Some details are given below of the tube design and operating particulars for an example of the embodiment of Fig. l:

cold cathode tube Dimensions of electrode 3 3mm.by5mm. Diameter of electrode I 1 mm. Spacing between electrode 2 and electrodes 3 and 4 1 mm.

4 Spacing between electrodes 3 and 4 2mm. Gas filling:

Neon 92% Hydrogen 7% Argon 1% Gas pressure 100 mm./Hg.

Operating conditions H. T. voltage 160 volts. Potential of anode 2 +160 volts. Voltage of source BI -20 volts. Voltage of source B2 -150 volts. Auxiliary cathode current 1 m. a. Peak output current from cathode 3 About 20 m. a. Maintaining voltage for electrodes 2 and 3 About 160 volts.

Input pulses Amplitude 80 Duration microseconds" 5 Repetition frequency kilocycles per second l0 It should be mentioned that the permanently discharging auxiliary cathode 4 is not essential and could be omitted, but it is desirable to provide it in some cases to control the ionisation base in the tube, in order to stabilise its operation.

While the principles of the invention have been described above in connection with specific embodiments and particular modiiications thereof, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.

What is claimed is:

1. An electric pulse time modulator comprising a cold cathode gas discharge tube having two electrodes with a gap therebetween, means for applying between the said electrodes a difference of potential less than the potential necessary for maintaining a discharge across the gap, means for applying a train of pulses to one of the said electrodes, each pulse being of such amplitude asto raise the said dierence of potential to a value greater than the potential necessary for producing a discharge across the gap, means for applying a modulating signal wave to one of the said electrodes to vary the said difference of potential and means for deriving time modulated pulses from one of the said electrodes.

2. A modulator according to claim 1 comprising an auxiliary discharge gap in the said tube, and means for maintaining a permanent discharge across the said gap.

3. A modulator according to claim 1 in which the said tube includes an anode and a cathode consisting of metal plates or rods arranged parallel to one another.

4. A modulator according to claim 3 comprising an auxiliary cathode in the form of a rod or wire arranged opposite the anode and spaced apart from the rst mentioned cathode.

5. A modulator according to claim 3 in which the train of pulses is applied to the said anode and in which the signal wave is applied to the first mentioned cathode.

6. A modulator according to claim 3, comprising a transformer having a primary winding connected to the rst mentioned cathode, and means for deriving duration modulated pulses from the secondary winding.

7. A modulator according to claim 6 comprising means for applying the duration modulated pulses to a differentiating circuit, in order to obtain pairs of differential pulses of opposite sign corresponding respectively to the leading and trailing edges of the direction modulated pulses, and means for eliminating the differential pulses corresponding to the said trailing edges. 5

GEORGE HUBERT HOUGH. THOMAS MEIRION JACKSON.

REFERENCES CITED The following references are of record in the 10 file of this patent:

Number Number 6 UNITED STATES PATENTS Name Date Rentschler Aug. 30, 1932 Percival et al Dec. 16, 1941 Cade Oct. 21, 1947 FOREIGN PATENTS Country Date Australia June 10, 1932 

